The three big differences between Rules Cyclopedia D&D and 1970s OD&D are:
1) RC D&D's 'universal stat bonus' where the same score grants the same bonus in every stat wasn't present in 1970s OD&D, which granted bonuses in a much more ad-hoc manner (more like 1E AD&D)
2) The idea of dwarfs, elves, and halflings as 'classes' wasn't present in 1970s OD&D -- sure in the original boxed set dwarfs and hobbits could only be fighting men and elves were automatically fighter/magic-users, but when Supplement I: Greyhawk introduced the thief class it was open equally to humans and demi-humans alike, so there was never a time in 1970s OD&D when a human could be a thief but a demi-human couldn't
3) RC D&D is based pretty much solely on the 1974 bozed set and about half the material from Supplement I: Greyhawk. There was a lot of additional material in the OD&D supplements (and Strategic Review and Dragon articles) that wasn't included in RC D&D that was included in 1E AD&D, including exceptional strength (18/xx), the magic-user chance to know spell and min/max spells per level table, half-elves, demi-human thieves and multi-classes, druids, paladins, rangers, illusionists, assassins, monks, bards, 2-axis alignment (good/evil in addition to law/chaos), the weapon vs AC chart and different damage values for size SM vs size L opponents, several spells, monsters, and lots of magic items, psionics, artifacts & relics, demons, the Outer Planes -- all of these were present in OD&D before it became AD&D, and none of them were carried over to RC D&D (or at least not in the same form -- for instance, RC D&D does have paladins and druids that were added in the Companion set in 1984, but they're very different from the OD&D classes which are more like the AD&D classes).
Other than these things, you'll find that RC D&D and 1970s OD&D (original boxed set + 1/2 of supplement I only) are pretty close to each other; closer than either is to 1E AD&D (which changed almost all of the numbers around -- xp progressions, spell progressions, character hit dice, the AC scale, the to hit and saving throw charts, etc. -- all of these are the same between 1970s OD&D and RC D&D, and all of them are different in 1E AD&D).